Electric motors are used for various technical applications. For example, in electric or hybrid vehicles, electric motors are used for driving a motor vehicle. Continuous and precise detection of the torque generated by the electric motor is necessary for the use of electric motors in motor vehicles, in order to be able to avoid inadvertent accelerations or decelerations of the motor vehicle. For this purpose, the torque is determined from the measured phase currents of the electromagnets of the electric motor and the rotation angle of the rotor of the electric motor using a machine model. The machine model contains operation-dependent parameters, e.g., the temperature of the stator and the rotor or the rotor flux, which must be complexly adapted. Errors or inaccuracies in the calculation path of the machine model result in incorrectly ascertained torques, which may have safety-relevant consequences for the motor vehicle. For this reason, there are methods for performing a plausibility check of the torque calculated using the machine model from measured values of the voltage and the current, the torque therefore being disadvantageously dependent on the precision and functionality of the sensors for these measured values.
A method for operating a variable-speed electric motor, in particular a brushless servo motor, is discussed in DE 41 22 391 A1. By regulating the motor variables current, position angle, and speed, the speed is determined from the position angle measured with the aid of a position sensor by a filter.